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Understanding Gold Plating

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Understanding Gold Plating Peter Wilkinson Engelhard Limited, Cinderford, Gloucestershire, United Kingdom For effective operation, gold plating baths must require the mini- mum of attention in respect of replenishment and replacement, and must produce as little sub-standard or reject work as possible. To achieve this, an understanding of the processes involved and the factors affecting them is advantageous. This article attempts to provide such an understanding in a simple manner. Special atten- tion is given to the plating of gold from acid baths containing monovalent gold complexes. The use of gold as a contact material on connectors, Other gold(I) complexes, such as those with thiomalate and switches and printed circuit boards (PCB's) is now well estab- thiosulphate have been considered, but it is doubtful if they lished. There is also an awareness that only gold deposits from will ever find commercial application. acid (pH 3.5-5.0) gold(I) cyanide baths containing cobalt, In terms of electricity consumed, gold(I) baths are more nickel or iron as brightening agents have the appropriate efficient than gold(III) baths. Thus, assuming 100 per cent properties for a contact material. Gold deposits from: pure efficiency, 100 amp-min will deposit 12.25 g of gold from a gold(I) cyanide baths; from such baths containing organic monovalent gold complex, but only 4.07 g from a trivalent gold materials as brightening agents; or from electrolytes containing complex. gold in the form of the gold(I) sulphite complex, do not have Nevertheless, deposition from acid gold(III) cyanide baths the sliding wear resistance which is necessary for make and has merit in the plating of acid resistant substrates such as stain- break connections (1). less steels, to which electrodeposited gold normally adheres It was known for many years that deposits from acid gold(I) badly because of their passivated surfaces. Hence, these baths cyanide baths brightened with cobalt, nickel or iron contained can also be used to produce decorative gold alloy coatings on about 0.25 per cent of base metal, but this did not explain why such substrates. their densities (16.0-17.0 g/cm 3) were so much lower than the density of pure gold deposits (19.3 g/cm 3). The classical exper- iment of Munier (2), in which she dissolved the gold of the The Deposition Process brightened deposits in aqua regia vapour to reveal a polymer Figure 1 is a schematic diagram of what is thought to happen network, showed that deposits of this type were much more during the deposition of a metal from a solution of one of its complex than was formerly thought. complexes. In what follows, an earlier account (3) of the factors in- The cathode attracts predominantly positive ions to a region volved in the formulation and operation of acid gold plating near its surface which is known as the Helmholtz double layer. baths is brought up to date. Brief accounts are also included of In addition, negatively charged complex ions, such as the the electrodeposition of gold from gold(III) cyanide baths and Au(CN)--, ions present in the gold(I) cyanide plating baths, from gold(I) sulphite baths. which approach this layer become polarised in the electric field of the cathode. The Formulation of Gold Electrolytes The distribution of the ligands around the metal is thereby Range of Electrolytes Available distorted and the diffusion of the complex ion into the Helm- There are only three complexes of gold which are suitable holtz layer is assisted. for commercial use in gold plating baths. By far the most im- Finally, within the Helmholtz layer the complex breaks up. portant are the cyanide and sulphite complexes of gold(I), of Its component ligand ions or molecules are freed and the metal which the potassium derivatives are KAu(CN), and IC 3Au(S0 3), released in the form of the positively charged metal cation respectively. which is deposited as the metal atom on the cathode. Gold Bull., 1986, 19, (3) 75 It will be appreciated from this simple picture of events at the cathode, that the ease of gold deposition from gold(I) cyanide baths, which contain gold in the form of Au(CN)2 ions, will depend on the ease with which these ions dissociate into Au + and CN ions, thus: Au(CN) Au+ + 2C1\1- 1 The mass action law dictates that for a reversible reaction: IStOJIK? [Au(CN) 2] = a constant 2 ] [Au+ ] [C1N- 2 ayer [Bulk * where [Au(CN)2], [Aul and [CN -] are the concentrations of the respective ions in solution. ELECTROLYTE This constant (4) provides a measure of the stability of the ess 2 Au(CN)2 complex and is referred to as its stability constant j3 •,•4gg404100 .ti5co4, (the subscript 2 indicates that the reaction is two stage). wslo?;:ig The value of 1 2 for the Au(CN)2 ion has been determined as 10383 . This is exceedingly large and implies that the equilibrium in reaction (1) is far to the left and that the Au(CN); ion is a very stable one. Moreover, it indicates that the concentration whereas in alkaline baths it is decreased. Despite the fact that + + ions is given by equation: [Au ] of Au the stability constant of the Au(CN)2 ion is unchanged, this complex ion therefore appears to be less stable in acid than in [Au(CN);] [Au+ 1 10-383 alkaline media. The range of usefulness of gold(I) cyanide ] [CN 2 baths in acid media is limited, however, by the tendency of the Au(CN)2 complex to decompose at pH's below about 3 with This is an exceedingly low concentration and it would appear precipitation of AuCN thus: as if reasonable rates of deposition of gold from gold(I) cyanide solutions are possible only because of the polarisation Au(CN)z —> AuCN + CN- of Au(CN); ions which approach the cathode surfaces and their decomposition in accord with equation./ within the Helmholtz Both polarographic and potentiometric studies have shown layer. that the gold(III) cyanide complex Au(CN) -4 is formed in acid The electrodeposition of gold from its other complexes is gold(I) cyanide plating baths during operation, presumably by thought to occur in a similar manner. oxidation at the anode (5,6). It is important to realise that any gold oxidised to the trivalent state in this manner is not avail- able for deposition at the cathode because gold(I) and hydro- Gold(I) Cyanide Baths gen are preferentially deposited. The efficiency of the bath These may be operated under alkaline, neutral or acid con- therefore falls, not because some gold is being deposited from ditions. Under acid conditions, the value of [Au + ]is increased as gold(III) cyanide, but because the effective concentration of a result of the equilibrium reaction: gold(I) cyanide in the bath has been reduced. Also gold(III) is just as susceptible to 'drag out' as the gold(I) and the metal's H+ + CN HCN valency does not affect its price. Modern acid gold(I) cyanide baths are therefore formulated in which the formation of undissociated HCN is strongly with the addition of a mild reducing agent so as to minimise the favoured. Acid gold(I) plating baths are therefore characterised formation of gold(III) cyanide, but baths formulated several by low free cyanide ion concentrations. However, under alka- years ago can give rise to gold(III) cyanide formation especially line conditions and especially if potassium cyanide is present, under unusual plating conditions, such as wire plating. Since the value of [CN1 is greatly increased. Following equations 1 to the gold(III) complex is also very stable, it is better to prevent 3 this means that in acid baths the value of [Au '] is increased, its formation than to reduce it back to gold(I) cyanide. 76 GoldBult, 1986, 19, (3) Gold(III) Cyanide Baths It must be stressed, however, that these baths are not used As has been indicated above, acid gold(I) cyanide plating commercially to any great extent. The deposits they yield can- baths are normally operated at pH values between 3.5 and 5. not be used in contact applications in electronics because their They cannot be operated at pH values less than 3.5, since under wear resistance is inadequate against existing gold plated con- such acid conditions the gold is precipitated as AuCN. More- tacts. Also the rates of deposition from them are slow so they over, deposits from acid gold(I) cyanide baths have little duc- cannot be used in modern high speed equipment. Moreover, tility so that plated articles cannot be subsequently deformed they are corrosive. without risk of cracks developing in the surface coating. These Apart from the chloride-free baths discussed above, other and other limitations of acid gold(I) cyanide baths have stimu- modern gold(III) baths are available commercially which do lated studies in recent years of the deposition of gold from contain chloride but which contain substances which prevent gold(III) cyanide baths. formation of chlorine at the anode. Thus, a method for the preparation of HAu(CN) 4 and the for- At present gold(III)/cobalt and gold(III)/nickel baths are mulation of gold plating baths based upon this gold complex used for plating stainless steel or chromium with flash coatings were described in 1971 (7). The baths were found to have ex- of gold (about 0.1 ,um thick). cellent plating characteristics at pH values from 0.1 to 5.0, and A gold(III)/indium bath has also been developed which gave optimum performance in the pH range 1.0 to 3.0.
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